We have isolated and cloned several sublines from individual virus-induced Balb/c mammary neoplasms. We have obtained evidence that MuMTV in some of these clones is hormone-inducible, but replication-defective. An MuMTV is activated in hormone-induced Balb/c neoplasms. This virus is also replication-defective. In the coming year we will determine the extent of and basis for the defective expression and if there is a relationship between its hormonal induction and altered control of growth associated with tumorigenicity. These studies will be conducted using clones of cells from tumor, grown either in methocel or collagen suspension culture.